a. Field of the Invention
The present invention relates to a thin film forming apparatus.
b. Description of the Prior Art
Recently, amorphous silicon (to be referred to as a-Si hereinafter) has been used as a material for a solar cell and a thin film transistor. Since a-Si has a photoconductive property, a variety of applications are being considered such as in a photosensitive film to replace an Se film and in a photoconductive element in a one or two-dimensional image sensor.
A thin film forming apparatus using glow discharge is known as a typical apparatus for forming a thin film such as an a-Si film. This apparatus is operated such that a compound gas is supplied to and discharged from a discharge chamber housing a substrate therein, and the compound gas is decomposed by glow discharge to deposit a thin a-Si film on the substrate. A compound for forming the a-Si film is represented by the general formula SiH.sub.x X.sub.4-x (wherein X is F or Cl, and x is an integer from 0 to 4). For example, a compound such as SiH.sub.4, SiF.sub.4, SiHF.sub.3, SiH.sub.3 Cl.sub.3, SiH.sub.2 Cl.sub.2 and Si.sub.2 H.sub.6 or a mixture thereof is used for this purpose. The compound described above is generally held in a gas phase and can be used alone or diluted with an inert gas (e.g., Ar, He or Xe) or a gas (e.g., H.sub.2). The thin film forming apparatus utilizing glow discharge is presently considered most effective as a means of forming a thin a-Si film.
However, the following problem is presented by an apparatus of this type. In deposition of the thin a-Si film, only a small proportion (%) of Si atoms contained in the compound gas is used for deposition of the thin a-Si film on the substrate. For this reason, a large amount of the compound gas is required to obtain a thin amorphous film having a desired thickness, resulting in high cost. In particular, when a relatively thick film having a thickness of 3 to 10 .mu.m is formed for a two-story two-dimensional image sensor, the utilization efficiency of the compound gas is degraded, resulting in a crucial problem.
In order to solve the above problem, a thin amorphous film forming apparatus has been recently proposed in Japanese Patent Disclosure No. 57-64922, wherein a plurality of discharge chambers are connected in tandem with each other, and a compound gas flows from the first discharge chamber to the second and subsequent discharge chambers. According to this apparatus, the compound gas used in the preceding chambers is reused in the subsequent chambers thereof, thereby improving the utilization efficiency of the compound gas. However, the amount of gas and its activated state in each discharge chamber vary, so the properties of films formed in the respective discharge chambers vary. As a result, it is difficult to uniformly prepare devices having thin amorphous films of uniform equality.